Fetcher类的工作流程

Fetcher类工作流程：

FileInputFormat.addInputPath(job, new Path(segment, CrawlDatum.GENERATE_DIR_NAME));
job.setInputFormat(InputFormat.class);

----------------第一部分------------------------
job.setMapRunnerClass(Fetcher.class);

Fetcher类实现了MapRunnable<Text, CrawlDatum, Text, NutchWritable>这个接口，主要完成了生产者的启动与消费者的启动。
Fetcher extends Configured implements Tool,MapRunnable<Text, CrawlDatum, Text, NutchWritable>
public void run(RecordReader<Text, CrawlDatum> input,OutputCollector<Text, NutchWritable> output,Reporter reporter){
	启动生产者
	feeder = new QueueFeeder(input, fetchQueues, threadCount * queueDepthMuliplier);
	启动消费者
	for (int i = 0; i < threadCount; i++) {       // spawn threads
      		new FetcherThread(getConf()).start();
    	}
}
FetcherThread类完成网页的下载，并用如下的方法多路输出。

----------------第二部分：多路输出------------------------
FileOutputFormat.setOutputPath(job, segment);
job.setOutputFormat(FetcherOutputFormat.class);/** Splits FetcherOutput entries into multiple map files. */
output.collect(key, new NutchWritable(datum));
output.collect(key, new NutchWritable(content));
output.collect(url, new NutchWritable(new ParseImpl(new ParseText(parse.getText()),parseData, parse.isCanonical())));